Since 2001, a number of intensive care units have adopted glycemic control protocols for the maintenance of glucose at close to physiological levels. The process of maintaining tight glycemic control requires frequent blood glucose measurements. The blood utilized for these measurements is typically obtained by procurement of a sample from a fingerstick, arterial line, or central venous catheter. Fingerstick measurements are generally considered undesirable due to the pain associated with the fingerstick process and the nuisance associated with procurement of a quality sample. Sample procurement from central venous catheters can also present problems since current clinical protocols recommend the stoppage of all fluid infusions prior to the procurement of a sample. Consequently, the use of arterial catheters has become more common. Arterial blood gas catheters are typically placed for hemodynamic monitoring of the patient, also referred to as real-time continuous blood pressure monitoring. These catheters are maintained for a period of time and used for both hemodynamic monitoring and blood sample procurement. Arterial blood gas catheters are not used for drug or intravenous feedings so issues associated with cross-contamination are minimized.
The process of procuring an arterial blood sample for measurement typically involves the following steps. The slow saline infusion used to keep the artery open is stopped and some type of valve mechanism such as a stopcock is opened to allow fluid connectivity to the mechanism for blood draw. The process of opening the stopcock and concurrently closing off fluid connectivity to the pressure transducer will cause a stoppage of patient pressure monitoring as the transducer no longer has direct fluid access to the patient. The sample procurement process is initiated. The initial volume drawn through the stopcock is saline followed by a transition period of blood and saline and subsequently pure blood. Generally, at the point where there is no or very little saline in the blood sample at the stopcock (or a knowable saline concentration), the measurement sample is obtained. The blood and saline sample obtained previously can be discarded or infused back into the patient.
In many intensive care units, a significant portion of blood samples obtained from arterial catheters are procured using blood sparing systems. In this process a leading sample containing both saline and blood is withdrawn from the patient and stored in a reservoir that lies beyond the sample acquisition port. A sample of blood that is free of saline contamination can then be procured at the sample port for measurement. Example embodiments of such blood sparing techniques include the Edward's VAMP system, shown in FIG. 1, and the Abbott SafeSet system. The Edward's VAMP in-service poster is incorporated by reference. Following procurement of an undiluted sample for measurement, the remaining blood/saline mixture can be re-infused into the patient. FIG. 1 is a schematic depiction of Edward's VAMP Plus System, an example blood sparing device. In the example device, a blood access system attached to arterial line, blood withdrawn and re-infused. A pressure monitoring transducer is remote from patient (60 inches). The tubing used between patient and pressure transducer is very stiff so compliance is minimized. A saline wash of transducer is provided after a clean sample is drawn into the syringe.
Air bubbles represent a significant problem for hemodynamic monitoring systems as they change the overall performance of the system. Air bubbles can become trapped in the monitoring system during filling, blood sampling, or added later by manual flushing or continuous flush devices. The presence of an air bubble adds undesirable compliance to the system and tends to decrease the resonant frequency and increase the damping coefficient. The resonant frequency typically falls faster than the damping increases, resulting in a very undesirable condition. FIG. 2 illustrates the effect of adding microliter air bubbles of various sizes to a transducer-tubing system. As more and more air is added to the system, the decrease in resonant frequency produces larger and larger errors in the systolic pressure, even though damping is increasing at the same time. Eventually, so much air could be added that the system produces only damped sine waves. Air bubbles diminish, not enhance, the performance of blood pressure monitoring systems. The preceding information was obtained from the Association for the Advancement of Medical Instrumentation, technical information report titled “Evaluation of clinical systems for invasive blood pressure monitoring”.
In clinical use, a pressure monitoring system should be able to detect changes quickly. This is known as its “frequency response”. The addition of damping to a monitoring system will tend to decrease its responsiveness to changes in the frequency of the pressure waveform but prevents unwanted resonances. This is especially so if changes are occurring rapidly such as occur at high heart rates or with a hyperdynamic heart. During these conditions it is essential that the system have a high “natural” or “untamed” frequency response. The optimal pressure monitoring system should have a high frequency such that over damped or under damped waveforms are unlikely regardless of the degree of damping present. The relationship of frequency and camping coefficient have been explored and defined by Reed Gardner. This relationship is well described in “Direct Blood Pressure Measurements—Dynamic Response Requirements” anesthesiology pages 227-23 6, 1981, incorporated herein by reference. FIG. 3 shows the resulting relationship between damping and natural frequency.
Due to the existing performance requirements and the fact that air bubbles dramatically alter the performance of a typical hemodynamic monitoring system, it is clinical practice to have the clinician evaluate the system carefully for the presence of any air bubbles. As stated by Michael Cheatham in “Hemodynamic Monitoring: Dynamic Response Artifacts” (available from www.surgicalcriticalcare.net), perhaps the single most important step in optimizing dynamic response is ensuring that all transducers, tubing, stopcock, and injection ports are free of air bubbles. Air, by virtue of being more compressible than fluid, tends to act as a shock absorber within a pressure monitoring system leading to a over damped waveform with its attendant underestimation of systolic blood pressure and over estimation of diastolic blood pressure. The identification of air bubbles is typically done by visual inspection of the system as well as by a dynamic response test. In practice this dynamic response test is achieved by doing a fast—flush test. A fast flesh or square wave test is performed by opening the valve of the continuous flush device such that flow through the catheter tubing is actually increased to approximately 30 ml/hr versus the typical 1-3 ml/hr. This generates an acute rise in pressure within the system such that a square wave is generated on the bedside monitor. With closure of the valve, a sinusoidal pressure wave of a given frequency and progressively decreasing implicated is generated. A system with appropriate dynamic response characteristics will return to the baseline pressure waveform within one or two oscillations, as illustrated in FIG. 4. If the fast—flush technique produces dynamic response characteristics that are inadequate, the clinician should troubleshoot the system to remove air bubbles, minimize tubing junctions, etc., until acceptable dynamic response is achieved.
In almost any automated blood glucose monitoring system, the device must procure or withdraw a sample of blood from the body. This process may require a few milliliters of blood or only a few micro liters. Regardless of the amount, the process exposes the associated fluid column to pressure gradients, potentially different pressures and fluid flows. Therefore, the process of procuring a blood sample has the potential to create bubbles within the fluid column. The fluid column is not intended to be restrictive but to apply to any of the fluid associated with the automated sample measurement system. Solubility is the property of a solid, liquid or gas called solute to dissolve in a liquid solvent to form a homogeneous solution. The solubility of a substance strongly depends on the used solvent as well as on temperature and pressure. In the application of automated blood measurements, the liquid solvent is blood, saline or any intravenous solution. The solute is air, oxygen or any gas in the liquid solvent. Changers in solubility due to temperature or pressure may result in bubble formation. As a solution warms it will typically outgas due to a decrease in solubility with temperature. Changes in pressure can also result in bubbles. The solubility of gas in a liquid increases with increasing pressure. Henry's Law states that: the solubility of a gas in a liquid is directly proportional to the pressure of that gas above the surface of the solution. If the pressure is increased, the gas molecules are forced into the solution since this will best relieve the pressure that has been applied.
Bubbles may be formed due to cavitation. Cavitation is the formation of bubbles in a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation can occur due to pumping at the low pressure or suction side of the pump. Cavitation can occur via multiple methods but the most common are vaporization, air ingestion (not always considered cavitation, but has similar symptoms), and flow turbulence
In a typical process of procuring a blood sample, a negative or reduced pressure is created so that the blood flows out of the body. This reduction in pressure creates an opportunity for bubble creation. Additionally, temperature differences between the human body, the ambient air, and any IV solutions also create the opportunity for bubble creation. Almost any form of pumping device creates some small degree of cavitation. Therefore, the process of attaching or combining a hemodynamic monitoring system with an automated blood measurement system creates the opportunity for bubble formation which in turn can result in poor performance of the hemodynamic monitoring system.